In ink-jet printers, an ink is delivered from an ink cartridge to a printer head by capillary action. However, it is known that, during delivery of the ink, minute air bubbles present in the ink or within the ink cartridge gives flow resistance to the ink. Moreover, it is also known that, when the ink head is repeatedly pressurized and depressurized during ink ejection, dissolved gases (e.g., dissolved oxygen and dissolved nitrogen) present in the ink tend to stagnate in the ink head and cause print dot losses at the time of ink ejection.
Methods for removing dissolved gases from an ink are roughly classified into physical methods for degassing an ink by physical means such as boiling or evacuation, and chemical methods for degassing an ink by introducing gas absorbents thereinto. However, physical methods are disadvantageous, for example, in that the degree of degassing is insufficient or the ink may be deteriorated. Moreover, chemical methods are also disadvantageous, for example, in that the dye contained in the ink may undergo a chemical change.
In order to solve these problems, Japanese Patent Laid-Open No. 17712/'93 discloses a method for the degassing of an ink for use in ink-jet recording which comprises passing the ink through hollow fibers comprising a gas-permeable hollow fiber membrane, and evacuating the outer surface side of the hollow fibers to remove dissolved gases from the ink by permeation through the hollow fiber membrane. The use of a hollow fiber membrane makes it possible to remove dissolved gases efficiently from an ink without exerting an adverse influence on the properties of the ink.
However, hollow fibers having an inner diameter of 20 to 30 μm are used in the ink degassing method disclosed in Japanese Patent Laid-Open No. 17712/'93. This causes a considerable pressure loss in the hollow fibers and hence requires high mechanical strength for the whole system, resulting in an increased cost. Moreover, the hollow fibers have a membrane thickness of 10 μm or less. This is disadvantageous in that, when the outer surface side of the hollow fibers is evacuated by starting a vacuum pump or returned to atmospheric pressure, the hollow fibers tend to vibrate and suffer damage as a result of mutual contact.
In order to improve their wetting properties and penetrating power into paper, inks for use in ink-jet printers usually contain a hydrophilic compound such as alcohol or ethylene glycol. Consequently, a method using a porous hollow fiber membrane can maintain high permeability to gases even if the membrane thickness is large. However, the surface of the porous base material is gradually made hydrophilic by the hydrophilic compound, so that the ink may undesirably leak out through the pores of the membrane. On the other hand, in a degassing method using a nonporous hollow fiber membrane such as one formed of teflon, the oxygen and nitrogen permeation fluxes of the teflon membrane are as low as 7.5 to 22.5×10−10 cm3/(cm2·Pa·sec). Consequently, when the hollow fiber membrane has a membrane thickness ensuring adequate mechanical strength, it is difficult to obtain a sufficient gas permeability. As a result, the dissolved gas concentration in the degassed ink has been limited to as high as 6.4 ppm or so.